ML20115G850

Non-proprietary Prairie Island Unit 1 Cycle 18 Thimble Deletion Study

ML20115G850
Person / Time
Site:	Prairie Island
Issue date:	07/12/1996
From:	Lesko J, Srinilta S WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML19311C094	List: ML20115G789 ML20115G812 ML20115G850 ML20115G880
References
NUDOCS 9607220091
Download: ML20115G850 (24)

Text

-_ _ _ ._, . - . .. ._.

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Exhibit E d

Prairie Island Nuclear Generating Plant

License Amendment Request Dated July 15,1996 I

Prairie island Unit 1 Cycle 18

, Thimble Deletion Study 1

NON-PROPRIETARY 4

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3 i

9607220091 DR 960715 ADOCK 05000282 .

PDR \

Wsstinghouse Non-Propri1 cry Class 3 i

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i PRAIRIE ISLAND UNIT 1 CYCLE 18  ?

1 THIMBLE DELETION STUDY  !

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C. C2n' Ab S. Srinilta 1 Core Analysis B Date: 7/i L!4 L '

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Verified:

J. R. Lesko <

Core Analysis A Date: 7/f t/4 h .

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.

• 19% Westinghouse Electric Corporation All Rights Reserved 1

I

( PRAIRIE ISLAND UNIT I AND UNIT 2 1

EVALUATION OF THIMBLE DELETION ON PEAKING FACTORS Introduction )

l A study utilizing Prairie Island Unit I and 2 flux maps was undertaken to assess incremental peaking l factor measurement uncertainties associated with a reductim to a minimum of 18 of the 36 of the movable detector (M/D) thimbles. Due to a large database used in the study, it is intended that the uncertainties quantified herein is to be considered of a generic nature and applicable to subsequent cycles.

Section 1 of this study presents the methodology and results of randomly deleting thimbles from actual INCORE maps to quantify the uncertainties. Section 2 quantifies the minimum number of thimbles per -

quadrant required in order to improve the ability to distinguish between random and systematic thimble l deletion events and to establish the bounds of applicability of Section 1.

For Prairie Island Unit 1 Cycle 18, an evaluation was performed to confirm applicability of this cycle j to the study described herein. Review of current cycle flux maps indicate that measurement to predicted 1 l

peaking factors are well within the required measurement uncertainties and indicate the core is behaving as predicted. Based on this, it is not anticipated that the core will not perform as expected for the l remainder of the cycle. It is not expected that the additional uncertainties on the peaking factors will result in any violation of the limits. Even with the increased measurement uncertainty applied as a result of the thimble detection study, the Prairie Island Unit 1 Cycle 18 FqSurveillance Technical Specification will provide necessary protection.

When referring to percentages in Sections 1 and 2, they refer to the percentage from a total of 36 '

' thimbles unless otherwise specified.

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1 2

SECTION 1 METHODOLOGY - GENERAL To assess the additional peaking factor measurement uncertainties associated with as few as 50% of the M/D thimbles available, ten full core INCORE flux maps from Prairie Island Unit I and Unit 2 (5 maps per unit) were used. The selection of these maps was made to cover the entire cycle burnup ranges. For each of the INCORE maps, five separate random deletions were made, giving a total of 50 thimble deletion cases with 50% of the thimbles available. Five separate random deletions were also done with this same set of 10 INCORE maps giving 50 thimble deletion cases with 75% of the thimbles available.

The INCORE code was used to randomly delete thimble locations. The measured peaking factors for the thimble deletion maps were then compared with the measured peaking factors in the reference maps, i.e.,

the INCORE maps employing all or most of the 36 movable detector thimbles. Figure I shows the movable detector (M/D) locations for the Prairie Island plants considered and Table 1 provides additional information on the 10 maps used in the study. For those maps with less than 100% of available thimbles (e.g., 80.6 %), thimble deletion cases were run deleting 50 % of the total 36 thimbles (e.g. 30.6%). These comparisons yielded the additional measurement uncertainties to be applied to3 F g and F q . Thimble deletion effects on the INCORE measured axial offset and quadrant tilt were addressed in a similar 1 manner. '

l METHODOLOGY - STATISTICAL l

~ l The percent error between the reference peaking factor value, Fa, (Reference), and the thimble deletion i case peaking factor value, Fa, (T.D.) is defined in Equation 1 as l l

a (T.D.)

% Error (T.D.) = 1- x 100 (Eq.1) r Fa (Reference),

where Fa is F33, or F qand T.D. refers to 75% or 50% of available thimbles in the reference case. A positive value of error implies that the peaking factor from the thimble deletion map is non-conservative j relative to the reference. In the following paragraphs the error will be denoted X; where i refers to one of the 10 flux maps and j refers to one of t19 5 thimble-deletion cases for each map. The percent error between the reference value and the thimble deletion case value for quadrant tilt and axial offset are defined in Equations 2 and 3 as Error (T.D.) = (Ref. - Deleted) x 100 for QUAD Tilt (Eq. 2) ,

i Error (T.D.) = (Ref. - Deleted) for A.O. (Eq. 3) i 3

(

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The mean error for map i, T; and the percent relative sample standard deviation for map i, S;, are  ;

defined in Equations 4 and 5, respectively.

l i

5 j

%= {X y j=1 (Eq. 4) )

' 5 A

{ (x, - 5)2 S, = I"I' (Eq. 5)

( 5-1  ;

1 i

After comput.ng Kand S; for eacl' smap, for each parameter of interest, and for both 50% and 75%

thimble deletion cases, the data is combined, The combined mean for all maps, Xcombined, is given by Equation 6 as:

10 '

1 E i (Eq. 6) l commat" i 3 =1EX l 1

i i

I 4

1 l

i The combined percent relative sample standard deviation of all maps is given by Equation 7 as: i l

" 10

{ ((N, - 1) S,' + N, {}

Sced=d * #

, Nr ,

N-I r  ;

1 where:

N; = Number of random deletion cases of each map = 5 and N7 = Total number of datapoints = 10 maps x 5 deletions / map = 50

- Equations 6 and 7 are constructed in such a manner that if one were to directly compute the mean and standard deviation for all 50 datapoints, the same numeric resuits would be obtained.

After im and Scombw have been obtained for each parameter of interest, and for both 50% and 75% thimble deletion cases, 95% confidence / 95% probability one-sided upper tolerance limits are constructed to quantify the thitrble deletion uncertainty component (See Equation 8).

l Thimble Deletion Uncertainty Component (%) = Tw + kScombin.d (Eq. 8)'

where k = the one-sided 95% confidence /95% probability tolerance limit factor for 49 degrees of freedom = 2.065.

Application of the above methodology is presented in the "Results" section of this report. The statistical combination of the thimble deletion uncertainty component with INCORE measurement is discussed in {

the " Thimble Deletion Uncertainty" section of this report.  !

RESULTS Table 2a provides the peaking factors sample mean (%) for each map (see Equation 4) and the sample standard deviation (%) for each map (see Equation 5) for the 50% thimbles available case. The combined sample mean (%) and the combined standard deviation (%) for each parameter of interest, as calculated per Equations 6 and 7, is also shown. Table 2b presents the analogous information for the 75% thimbles available case. Tables 2c and 2d provide the sample mean and the sample standard deviation for quadrant tilt and axial offset over the same database.

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D Thimble deletion uncertainty components (i.e. the 95% probability,95% confidence tolerance limit) for F3g,and Fq are calculated in Appendix A using Equation 8 and are based upon the data of Tables 2a and 2b. The Thimble Deletion Uncertainty Component (%) is plotted in Figure 2 as a function of Percentage of Thimbles Available. This figure is provided for information only and is not directly used in the uncertainty application.

THIMBLE DELETION UNCERTAINT.Y Current flux map peaking factor measurement uncertainties include allowance for down to 75 % thimbles available. Accordingly, an incremental thimble deletion uncertainty component penalty from 75% to 50%

of thimbles available could be considered to be appropriate. However, for conservatism and simplicity, the full thimble deletion uncertainty component penalty from 100% to 50% thimbles available will be used. The Thimble Deletion Uncertainty Component (50% T.D.) discussed in the preceding section is combined with the appropriate flux map measurement uncertainty to obtain a total uncertainty, F3a UNCERTAINTY, F3g u The appropriate equation for combining statistically independent uncertainty components is F[u(50%) = 1 + FAHro. m + ((F[y" - 1)2 + (KS)2rof2 (Eq. 9)

For conservatism, a negative value of T.D. Bias will be treated as zero. Analogous equations apply to Fqu. Evaluating the above expression yields the following result (a,c)

For conservatism to support generic application to subsequent cycles and to support Unit i Cycle 18, Fgu 3 (50%) will be rounded up to 1.06. This value can be interpreted as a 95% probability tolerance limit at a high confidence level. This two percent incremental thimble deletion penalty is linearly applied from 75% to 50% thimbles available (i.e.,1.04 at 27 thimbles and 1.% at 18 thimbles available).

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FqUNCERTAINTY, Fqu l (a.c)  !

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For conservatism to support generic application to subsequent cycles and to support Unit 1 Cycle 18, Fq u (50%) will be rounded to 1.08. This 3% incremental thimble deletion penalty is linearly applied from 75% to 50% thimbles available (i.e.,1.05 at 27 thimbles and 1.08 at 18 thimbles available).

AXIAL OFFSET AND QUADRANT TILT The mean change in quadrant tilt with 18 of the thimbles available was found to be only [

]*

• Similarly, the mean change in axial offset with 50% of the thimbles available was also quite small at [

]*

• Note that all uncertainties on A.0. and tilt are absolute values and not percentages of A.0. nor tilt. These values indicate that thimble deletion has a negligible impact on the core average axial power shape measur'ement.

' Changes of this magnitude are not significant and will not adversely affect excore detector calibration.

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CONSERVATIVE ASSUMPTIONS For convenience a summary of conservative assumptions employed in this study are provided below:  !

1) The total thimble deletion penalty from 100% to 50 % of the available thimbles was utilized rather  ;

than the incremental penalty from 75% to 50% of the available thimbles.

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2) . Thimble deletion uncertainty results were rounded up and negative bias values were set to zero. -

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5) [

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8 s-

e SECTION 2 This section quantifies the number of thimbles per quadrant required for Prairie Island Unit 1 in order to improve the ability to distinguish between random and systematic thimble deletion events and to establish the bounds of applicability of the incremental peaking factor uncertainties. He peaking factor measurement uncertainty analysis described in Section 1 makes the assumption that thimbles were ~

randomly deleted from the core. If thimbles are somehow systematically deleted from the core then the calculated peaking factor measurement uncertainties will not apply.

The assumption of random deletion of thimbles is an important one. If removal of instrumentation thimbles in the core is completely random then each thimble in the core has an equal probability of being removed from operation. Therefore, if 50 percent of the thimbles in the core were to be deleted randomly, a random pattern of thimbles would r-It. On the other hand, if there were some function driving the removal of the thimbles the result wot." - , be a random pattern of thimbles. This systematic deletion of thimbles could conceivably result in lai,,e areas of the core being uninstrumented.

The current Technical Specification requirement of a minimum of 2 M/D thimbles per core quadrant is not sufficient to distinguish between random and systematic deletion events with high confidence. To help insure that thimble deletion is random, a restriction can be placed on the number of thimbles that must remain operable in each quadrant. By defining the quadrant in such a manner as to essentially place a requirement on each 1/8th core, the ability to distinguish between random and systematic events will be significantly enhanced.

If, for example, for 50% thimbles remaining, the requirement of 2 or more thimbles per quadrant is satisfied, de'n in all likelihood a random deletion occurred and incremental thimble deletion peaking factor measurement uncertainties are appropriate. On the other hand, if there are less than two thimbles per quadrant, then it is possible that a systematic thimble deletion occurred and that the impact on measured quadrant peaking factors, may be larger than quantified in Section 1.

l 9

e METHODOLOGY - COMPUTER SIMULATION A short computer program for determining the probability distribution of thimbles remaining was written.

The program allows for different number of thimbles per quadrant and keeps track of interior, axis, and diagonal thimbles (see 2-loop description). This program has been used to determine the number of thimbles per quadrant for all of Westinghouse Thimble Deletion Analyses.

Starting with n7 thimbles in the core and randomly deleting down to r7 thimbles constitutes one case.

After deleting nr - r thimbles e from the core, the number of thimbles remaining in each of the eight quadrants is determined. The minimum number of thimbles remaining over all 8 quadrants is then found.

A large number of cases is run in order to determine the probability distribution of thimbles remaining.

2-LOOP PROBLEM DESCRIPTION The maximum possible number of available thimbles for a 2-loop Westinghouse PWR is 36. The initial distribution of these thimbles is provided in the following table. Figures 3 and 4 should also help in visualization.

No. of Interior Thimbles in Q1 6 No. of Interior Thimbles in Q2 7 No. of Interior Thimbles in Q3 7 -

No. of Interior Thimbles in Q4 6 No. of Axis Thimbles Ql-Q2 1 No. of Axis Thimbles Q2-Q3 3 No. of Axis Thimbles Q3-Q4 3

+

No. of Axis Thimbles Q4-Q1 _3_

36 Total No. of Interior Thimbles in QA 7 No. of Interior Thimbles in QB 8 No. of Interior Thimbles in QC 7 No. of Interior Thimbles in QD 8 No. of Diagonal Thimbles QA-QB 2 No. of Diagonal Thimbles QB-QC 2 No. of Diagonal Thimbles QB-QD 1 No. of Diagonal Thimbles QD-QA _1 36 Total Note that all thimbles are counted as whole values even if they lie on an axis or diagonal. Provided the technical specification value and computer simulation are consistent this is appropriate. Eighteen (18) thimbles are randomly deleted from-each case.

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e 2-LOOP PROBLEM RESULTS A 1000 case simulation was run to obtain the probability distribution of the minimum number of thimbles left after having reduced to 50% and 60% of the thimbles available. Results are summarized in Table 3.

[

]** Therefore, a requirement that 2 or more thimbles per quadrant for 50% be available is appropriate. Assuming random thimble deletion, it is unlikely that with 18 thimbles remaining overall, fewer than 2 thimbles will be available over the 8 quadrants.

CONCLUSION With the inclusion of the additional peaking factor uncertainties, it is concluded that operation of the

. movable detector system with a minimum of 50% of the thimbles available is acceptable provided that.

an additional 2.0% for F3s and 3.0% for F be q applied to the INCORE measured peaking factors.

However, when fewer than 75% of the thimbles are available there should be a minimum of 2 thimbles per quadrant where quadrant includes both horizontal-vertical quadrants and diagonally bounded quadrants. This requirement increases the ability to distinguish between random and systematic thimble deletion events. In addition, the confidence on the appropriateness of the incremental thimble deletion peaking factor uncertainty values is increased provided that 2 or more thimbles per quadrant are observed to be available, and counting thimbles on the axis and diagonal as whole values.

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TABLEI INCORE DETECTOR THIMBLE REDUCTION STUDY MAPS Burnup Core Power Percent (MWD /MTU)  % Thimble Available (Ref.) .

Unit 1 Cyc 15 MAP 6 210 100.0 94.4 MAP 8 2l00 100.0 94.4 MAP 13 7380 100.0 94.4 MAP 21 11590 100.0 94.4 MAP 25 15750 100.0 94.4 Unit 2 Cyc 15 MAP 6 304 100.0 80.6 MAP 8 2489 100.0 83.3 MAP 19 8545 100.0 77.8 MAP 24 13643 100.0 80.6 MAP 28 17734 100.0 80.6 e

9 12.

a-TABLE 2a SAMPLE STANDARD DEVIATION AND MEAN FOR INCORE MAPS WITH 50% OF THE THIMBLE AVAILABLE FOR TWO LOOP REACTOR CORE PARAMETERS F AH Fq Unit Cycle MAP {(%) S;(%) X,(%) Si (%)

I 15 6 (a,c) l~ 15 8 1 15 - 13

1. 15 21 -

1 15 25 2 15 '6 2 15 8 2 15 19 2- 15 24 2 15 28 S comb Ecomb '

13'

,r TABLE 2b 1

' SAMPLE STANDARD DEVIATION AND MEAN FOR INCORE MAPS WITH 75% OF THE THIMBLE AVAILABLE FOR TWO LOOP REACTOR CORE PARAMETERS Fg3 Fq l Unit Cycle MAP X,(%) S; (%) X,(%) S;(%)

d

, 1 15 6 (a,c) -

. 1 15 8 l 1 15 13 1 15 21 i 1 15 25

, 2 15 6

2 15 8 2 15 19 2 15 24 2 15 28 i

b comb i

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TABLE 2c SAMPLE STANDARD DEVIATION AND MEAN FOR INCORE MAPS WITH 50% THIMBLES AVAILABLE FOR TWO LOOP REACTOR CORE PARAMETERS

. A.O.* QUAD TILT +

Unit Cycle MAP X,(%) . Si (%) X,(%) Si (%)

1 15 6 (a,c) 1 15 18  ;

1 15 13 l

' 1 15 21 i 1 15 25 2 15 6 2 15 8 2 15 19 2 15 24

.2 15 28  :

S comb Ecom  !

i

+ Standard deviation for QUAD TILT about Atilt = (Ref. - Deleted) x 100%.

Standard deviation for A.O. about AA.O = (Ref. - Deleted).

15

TABLE 2d SAMPLE STANDARD DEVIATION AND MEAN FOR INCORE MAPS WITH 75% OF THE THIMBLE AVAILABLE FOR TWO LOOP REACTOR CORE PARAMETERS A .O.* QUAD TILT +

Unit Cycle MAP X,(%) S; (%) 2,(%) S;(%)

1- 15 6 (a,c) 1 15 8 1 15 13 1 15 21 1 15 25 2 15 6 2 15 8 2 15 19 2 15 24 2 15 28 S comb comb

+ Standard deviation for QUAD TILT about Atilt = (Ref. - Deleted) x 100%.

Standard deviation for A.0. about AA.0. = (Ref. Deleted).

a 0

16

. . . . - -. .. - . . - - . - . _ = - .- . _ = _ . .

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TABLE 3  !

l 2-LOOP CORE

SUMMARY

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1000 CASE THIMBLE DELETION SIMULATION '

' 50% and 60% THIMBLES AVAILABLE 12 cases at @%

Thimbles remaining # of case % cases Cumulative % # of cases % cases Cumulative %

(a,c)

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FIGURE 2 THIMBLE DELETION UNCERTAINTY COMPONENT VERSUS PERCENTAGE OF THIMBLES AVAILABLE (a,c)

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l APPENDIX A ]

i THIMBLE DELETION UNCERTAINTY COMPONENTS 95% PROBABILITY AND 95% CONFIDENCE (X,, + KScomb.)  !

_ 1 NORMAL (TYPICAL) FLUX MAPS l d

Fag Thimble Deletion Uncertainty Component 4

(a,c) '

i

} FqThimble Deletion Uncertainty Component l

i (8,c) i I +

l f-l i

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l 5

22

.. ~ . . - _ -. .- - . - . - . . _ . . _ . . . . - . ~ - . . - - . - . - - - _ . . . - . . . . .

1 4

APPENDIX B i

. TWO-SIDED 95% CONFIDENCE LIMITS ON MEAN ATILT AND MEAN AA.O.

I T,,, i t.025 smb. / /# (approximate t by z) i tilt or tilt or i A.O. A.O. .

i 4

Quadrant Tilt Uncertainty Component i

1 j (a,c) 1 1

} Axial Offset Uncertainty Component (a,c) 1 4

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Exhibit F i

j Prairie Island Nuclear Generating Plant License Amendment Request Dated July 15,1996

, Westinghouse Authorization Letter, CAW-96-987, and Accompanying Affidavit

. Proprietary information Notice l Copyright Notice i  !

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